The invention relates to a method and apparatus for determining in situ the earth's thermal conductivity and thermal capacity per unit volume.
The thermal properties of earth are of considerable importance to the geologist, construction engineer, and others engaged in the study and application of the earth sciences. Thermal conductivity and diffusivity of the earth are the principal determinants of the temperature profile of the earth's surface, the depth of frost penetration, the freezing and thawing characteristics of land areas, and similar factors which critically influence the design of buildings, airports, and roads, especially in arctic environments. The thermal properties of building construction and insulation materials must also be considered and evaluated to establish the overall heat flow pattern into the surrounding earth.
Thermal property measurements are often required in field locations as well as in the laboratory. Laboratory studies can be conducted under closely controlled conditions where equipment and personnel are not limited by the available power, space and an adverse environment. In situ measurements, however, must be conducted with few operating personnel and with limited portable equipment capable of operation in adverse environments.
The practice has been to measure the earth thermal conductivity in a borehole by means of a probe which consists of a body containing at least one temperature sensor and a heater. The sensor is thermally insulated from a heater and is adpated to be maintained in thermal contact with the wall of the borehole, cased or uncased, while in use. The sensed temperature of the surrounding earth will increase proportionally to the heat flux applied, and inversely proportional to the thermal conductivity of the earth. Consequently, by making temperature measurements at predetermined time intervals, a heating curve is obtained that may be related to thermal conductivity. Representative of this technique is a system described in U.S. Pat. No. 3,668,927.
An alternative technique described in U.S. Pat. No. 3,864,969 is to heat the earth for a predetermined period to elevate its temperature, and then log the rate of temperature decay. Still another technique described in U.S. Pat. No. 3,981,187 is to lower a heated probe in a borehole at a constant rate. A sensor at the leading end of the probe measures earth temperature before being heated, and a sensor at the trailing end of the probe measures earth temperature after heating. Thermal conductivity of the earth at any point is inversely proportional to the temperature change as the probe passes the point if the heat flux rate is maintained constant.
In all these prior art systems, the probe includes some means for maintaining physical contact of the probe with the wall of the borehole in order for the temperature sensors to be thermally coupled to the surrounding earth. Because the wall of the borehole is not necessarily smooth and straight, it is difficult to provide for the desired physical contact without some risk of the probe becoming stuck at some level as it is lowered, or jamming in the borehole as it is raised. There is therefore a need for a new probe and method of measuring thermal conductivity which does not require contact with the borehole wall.